1. Field of Invention
The invention relates to a phase demodulator, a symbol timing recovery circuit and the method thereof. More particularly, it relates to a phase demodulator, a symbol timing recovery circuit and the method thereof in a TDMA system using the π/4-DQPSK base frequency modulation technology.
2. Related Art
In digital radio baseband modulation technologies, the π/4-DQPSK baseband modulation technology is a common technique. It is widely used in the USDC and PACS systems in North America and the PDC and PHS systems in Japan. In addition, the π/4-DQPSK baseband modulation technology is usually performed using a phase demodulator. A conventional phase demodulator 1 is shown in FIG. 1. It has a symbol timing recovery circuit 15. As shown in the drawing, the phase demodulator 1 further includes a radio frequency (RF) circuit 11, an analog-to-digital (A/D) converter 12, a matched filter 13, and a phase difference generation circuit 14.
The RF circuit 11 receives an analog high-frequency signal and converts it into an analog intermediate-frequency signal. The analog intermediate-frequency signal is then converted by the A/D converter 12 and filtered by the matched filter 13 to produce an in-phase signal I and a quadrature signal Q. In general, the in-phase signal I and the quadrature signal Q are signed digital signals. The phase difference generation circuit 14 uses the in-phase signal I and the quadrature signal Q to obtain a phase difference Δθ. The symbol timing recovery circuit 15 then performs the symbol timing recovery using the phase difference output from the phase difference generation circuit 14.
As described before, the conventional symbol timing recovery circuit 15 computes an optimal sampling point of a symbol. Usually, the phase difference generation circuit 14 is used to obtain a phase difference Δθ for the symbol timing recovery circuit 15. The symbol timing recovery circuit 15 then uses the phase difference Δθ to compute an optimal sampling point and thereby recover the symbol timing. Such techniques can be seen in the U.S. Pat. No. 4,941,155. However, the drawback in that patent is that when obtaining the optimal sampling point, several mathematical conversion operations have to be done between a polar coordinate system and the I-Q orthogonal coordinate system. This will results in long execution time.
In light of the foregoing drawbacks, how to simplify operations in obtaining the optimal sampling point while at the same time shortening the operation time is an important subject. Moreover, how to simplify the optimal sampling point operation and the required circuit to simplify the phase demodulator is also an important issue under study.